1. Field of the Invention
The present invention relates to an image forming apparatus and a control method therefor.
2. Description of the Related Art
Conventionally, there has been known an electrophotographic image forming apparatus. The electrophotographic image forming apparatus has a laser light source for scanning (irradiating) a photosensitive member, i.e. a photosensitive drum with a laser beam corresponding to image data, and an electrostatic charger for electrically charging the photosensitive drum. Further, the electrophotographic image forming apparatus is provided with a development device for toner-developing an electrostatic latent image formed on the photosensitive drum by irradiation of the electrically charged photosensitive drum with the laser beam. Furthermore, the electrophotographic image forming apparatus is provided with a transfer device for transferring a toner image, developed on the photosensitive drum by the development device, onto a sheet (recording sheet).
Ideally, in the above-mentioned electrophotographic image forming apparatus, when the photosensitive drum is scanned (irradiated) with a laser beam emitted based on one-line image data, a scanning line is formed into a straight line extending parallel with the axis of the photosensitive drum. However, due to mechanical error of the mounting position or the like of the laser light source or the photosensitive drum, a case can occur where the scanning line is not a straight line parallel with the scanning direction (i.e. the scanning line is curved). To correct such curving of the scanning line, there have been proposed techniques mentioned below.
A method has been proposed in which in a step of mounting an exposure unit into the body of an image forming apparatus, the degree of inclination of each scanning line is measured using an optical sensor and then the inclination of the scanning line is adjusted by mechanically tilting the exposure unit, whereafter the exposure unit is mounted into the body of the image forming apparatus (see Japanese Patent Laid-Open Publication (Kokai) No. 2003-241131).
Further, another method has been proposed in which the respective degrees of inclination and curving of a scanning line are measured using an optical sensor and then bitmap image data is corrected such that the inclination and curve of the scanning line are offset, whereafter an image is formed based on the corrected bitmap image data (see Japanese Patent Laid-Open Publication (Kokai) No. 2004-170755).
This method is an electrical method in which the scanning line curve is corrected by processing image data. Therefore, the method disclosed by Japanese Patent Laid-Open Publication (Kokai) No. 2004-170755 dispenses with mechanical adjusting members and an adjusting step during assembly, and hence can cope the curving of the scanning line at lower costs than the method disclosed in Japanese Patent Laid-Open Publication (Kokai) No. 2003-241131.
In a case where image data obtained by reading an original by an image reader or the like is processed in an image processing section, the image processing section necessitates a buffer for temporarily storing the image data. To reduce the capacity of the buffer, a method has been proposed in which the image data is divided into a plurality of rectangular image data blocks, and the divided image data is read out from a memory in units of rectangular image data blocks. This method makes it unnecessary to provide the image processing section with a buffer which is capable of storing all pixels in a line direction of image data obtained by reading an original, and hence it is possible to reduce the capacity of the buffer.
However, if the image forming apparatus including the image processing section that processes image data by dividing the same into rectangular image data blocks is provided with the above-mentioned function for offsetting a scanning line curve, trouble can occur.